Preserving insect enemies that reside in your garden will help control some insect pests, but sending additional enemies into the area may not be so useful. Many gardeners buy packages of "beneficial insects"—insects that feed on pests. Then they release the beneficial insects into the garden to control aphids and other crop-destroying insects. Most of the time people get much less out of these releases than they expected.

Three of the most common beneficial insects sold for this purpose are lady beetles, praying mantises and green lacewings. Two of these insects often do not do much good.

A common problem with mail-order lady beetles is that they fly away soon after being released. Because most of them will not stay in a backyard garden, only community-wide releases are likely to provide much benefit. Praying mantises are fascinating to watch, but they are not useful or efficient predators. Putting green lacewing eggs in the garden is really the most promising strategy, and even that has questionable value.

Already established populations of beneficial insects are likely to provide better results than introducing purchased insects. The following are some simple principles to help preserve any beneficial insects already living in your garden.

§ Learn how to recognize beneficial insects.

§ Minimize insecticide applications. Most insecticides kill beneficial insects along with the pests. For example, microbial insecticides that contain different strains of the bacterium Bacillus thuringiensis are toxic only to caterpillars, certain beetles or certain mosquito and black fly larvae.

§ Maintain ground covers, standing crops and crop residues. To survive the winter, many beneficial insects require the protection offered by vegetation. However, preserving ground covers and crop residues can also help certain pests. Evaluate this strategy according to its overall benefits and drawbacks.

§ Provide pollen and nectar sources or other supplemental foods. Plants with very small flowers make good sources of nectar for adults of certain beneficial wasps. Seed mixes of flowering plants intended to attract and nourish beneficial insects are sold at garden centers and through mail-order catalogs.

When attempting to preserve beneficial insects, keep in mind that these natural enemies will never completely eliminate a pest. When a pest population becomes low, the beneficial insects often leave the area and search for more abundant prey.
If you need 100 percent control, natural enemies alone usually do not provide enough control. However, natural enemies will reduce the pest population to moderate levels, which are acceptable in many cases. One other point to remember is that natural enemies take time to do their work. Insecticides have nearly immediate effects on pest populations, but natural enemies need time to search for prey or hosts.